I believe there might be a bug in the Markdown preview script. It shows the following
But I've looked at several other Markdown parsers, including what could be considered the reference implementation at Daring Fireball and they render it as
which is what I would expect.
I'm not sure if the server-side implementation has this behaviour, so I'll put
foo**strong text** below to see what happens:
If this is by design then I'm sorry. If it's unintentional I'll see if I can make a patch.
EDIT: The above at the time of posting renders as
foo*<i>strong text*</i>. This is consistent with the behaviour of the previewer.